The present invention relates to digital signal measurements, and more particularly to a time correlated display of signal power to distortion characteristics for a time-variant signal.
Traditional swept spectrum analyzers measure the characteristics of a signal by sweeping a frequency band of interest across a fixed receiver circuit. This results in an average power measurement for each point in the display, which is then integrated over sub-bands of interest. The sub-bands of interest may represent channel power and adjacent channel power measurements for a multi-channel communication signal. This method results in measurement variations and inaccuracies due to the time-varying nature of the signal being measured. Since distortion products from a power amplifier or other non-linear equipment are directly related to the amount of input signal power, it is critical for an amplifier manufacturer to measure both signal power and signal distortion characteristics over a common time interval. New models of spectrum analyzers, such as the Tektronix RSA and WCA Series spectrum analyzers, the Agilent PSA Series spectrum analyzers and the Rohde & Schwartz FSQ Series signal analyzers, either have or soon may have the ability to measure an entire band of interest in a single time-domain acquisition and perform ACLR (Adjacent Channel Leakage Ratio) and ACPR (Adjacent Channel Power Ratio) measurements mathematically. These measurements are the ratio of the power in the main signal to the power appearing in some adjacent signal band. However these new models do not analyze correlated values for other measurements, or provide a display of the time period over which the ACLR/ACPR was measured.
In addition to ACLR/ACPR, PAR (Peak to Average Ratio) and CCDF (Complementary Cumulative Distribution Function) measurements are important. The PAR measurement is made by forming a ratio of the peak input signal power to the average power over a specified time interval. The CCDF (Complementary Cumulative Distribution Function) measurement represents the probability that the peak power above average power of input signals exceeds a threshold. PAR and CCDF are measured by existing instruments and one instrument, the Agilent 89600 Series, can correlate PAR and CCDF over the same acquisition window. However this instrument does not relate these results to the ACLR/ACPR measurement. Other measurements, including code domain power (CDP)—the power contained in each of several orthogonally coded signals within an RF carrier—and EVM (Error Vector Magnitude)—the calculation of the length of an error vector resulting from the modulation of a carrier—are also used in conjunction with ACLR/ACPR to “trade off” performance in one domain vs. performance in another. In other words current instrument do not allow these measurements to be time correlated to ACLR/ACPR. The result is that measurement of characteristics of non-linear RF products in response to a known test signal require looking at different screen displays, each screen display representing a different measurement based upon a different time interval. The present instruments only allow the measurements to be observed time sequentially.
As signal characteristics change with time, ACLR/ACPR and other measurements on separate sets of data result in poor repeatability of the measurements. If the PAR of the signal is high during measurement of the adjacent channel and low during the measurement of the main channel, an artificially low ACPR measurement results. If these conditions are reversed in the next measurement, an artificially high figure is measured. Since the ACLR/ACPR measurements are not correlated to the PAR and CCDF measurements, results cannot be compared between successive measurements. The effects of signal variations over time may be reduced by applying averaging techniques to the data, but this creates unacceptably long measurement times and only reduces the inaccuracy—it does not eliminate it.
What is desired is a measurement of signal to distortion characteristics that results in higher accuracy, greater repeatability and faster measurements than the current uncorrelated measurement methods by looking at multiple measurements simultaneously.